Clarify MSVC-only bundled SDL support

BUILDING.md

@@ -12,6 +12,7 @@ Requirements:
 - CMake 3.21+
 
 The repository already contains the required prebuilt Windows SDL packages in `Libraries/`.
+Those bundled packages are intended for **MSVC / Visual Studio**, not MinGW.
 
 Configure and build:
 
@@ -38,6 +39,14 @@ cmake --build build
 ### 3. Legacy Visual Studio solution
 The existing `.sln` and `.vcxproj` files are still in the repo and can still be opened directly in Visual Studio.
 
+### CLion with MinGW on Windows
+This is a different toolchain from MSVC. The bundled SDL packages in `Libraries/` are the Visual C++ SDL package, so they should not be used with MinGW.
+
+Use one of these options instead:
+
+- switch CLion to a Visual Studio toolchain
+- or install MinGW-compatible SDL packages and configure CMake with `-DMOTHERLOAD_USE_BUNDLED_WINDOWS_SDL=OFF`
+
 ## Build output
 
 The CMake build writes the executable to:
@@ -55,6 +64,8 @@ CMake uses the bundled SDL package config files from:
 - `Libraries/SDLMixer/.../cmake`
 - `Libraries/SDLTtf/.../cmake`
 
+These are only used for **MSVC** builds.
+
 ### Linux
 CMake falls back to `pkg-config` and expects these packages to be installed:
 - `sdl2`
@@ -76,6 +87,14 @@ sudo apt install libgl1-mesa-dev
 ### CMake cannot find SDL packages on Linux
 Install the SDL development packages listed above.
 
+### CLion / MinGW on Windows fails while linking SDL2main
+That usually means the Visual Studio SDL package from `Libraries/` is being used with MinGW.
+
+Use one of these fixes:
+
+- use a Visual Studio toolchain in CLion
+- or configure CMake with `-DMOTHERLOAD_USE_BUNDLED_WINDOWS_SDL=OFF` and provide MinGW-compatible SDL libraries
+
 ### The game starts but cannot find assets
 Run the executable from the build output directory, not from the source directory root. The post-build step copies `Resources/` next to the executable.